There is an increasing need for rapid, small scale and highly sensitive detection of biological molecules in medical, bioterrorism, food safety, and research applications. Nanostructures such as silicon nanowires and carbon nanotubes display physical and electronic properties amenable to use in miniature devices. Carbon nanotubes (CNTs) are rolled up graphene sheets having a diameter on the nanometer scale and typical lengths of up to several micrometers. CNTs can behave as semiconductors or metals depending on their chirality. Additionally, dissimilar carbon nanotubes may contact each other allowing the formation of a conductive path with interesting electrical, magnetic, nonlinear optical, thermal and mechanical properties.
It is known that single walled carbon nanotubes are sensitive to their chemical environment, specifically that exposure to air or oxygen alters their electrical properties (Collins et al. (2000) Science 287:1801). Additionally, exposure of CNTs to gas molecules such as NO2 or NH3 alters their electrical properties (Kong et al. (2000) Science 287:622). Thus chemical gas sensors can be designed, based on how they influence the electrical properties of carbon nanotubes such as described in DE10118200.
Detection of biomolecules has been achieved using probes that are attached to nanotubes or silicon nanowires. For example, a device using peptide nucleic acid receptors, designed to recognize a specific DNA sequence and attached to the surface of silicon nanowires, was able to detect the presence of a DNA sequence through hybridization-induced conductance changes (Hahm and Lieber (2003) Nano Lett. 4:51). Hybridization of a single stranded DNA probe attached to silicon nanowires with the complementary DNA strand was detected by conductance changes (Z. Li et al. (2003) Nano Lett. 4:245). In these two cases detection depends on the nanowires behaving as field effect transistors where changes in nanowire conductance result from binding of the target DNA to its complement, directly at the nanowire surface.
Hybridization of a single-stranded polyC DNA probe attached to carbon nanotubes with the complementary polyG DNA strand was detected amperometrically. (J. Li et al. (2003) Nano Lett. 3:597). In this case the oxidation of Ru(bpy)32+ was mediated by the guanine bases of the DNA, attached by hybridization to the CNTs.
In WO 02/48701, articles are described that use nanowires, including CNTs, to detect different types of analytes including biological analytes. The nanowire may be modified by attaching an agent that is designed to bind an analyte, the binding to the nanowire or to a coating on the nanowire then causes a detectable change in conductance. In this detection system, the interaction between the binding agent and the analyte to be detected alters the electrical conductance of the nanowire. This requirement in turn limits the functional location of the binding agent with respect to the nanowire in that they must be in close proximity, 5 nanometers or less.
Carbon nanotubes have been used in electrocatalysis. Microelectrodes, constructed of multiwalled carbon nanotubes, were shown to provide a catalytic surface for electrochemical reduction of dissolved oxygen, potentially useful in fuel cell applications (Britto et al. (1999) Advanced Materials 11:154). A film of single walled carbon nanotubes functionalized with carboxylic acid groups on a glassy carbon electrode showed electrocatalytic behavior with several redox active biomolecules, involving reduction of the carboxylic acid groups (Luo et al. (2001) Anal. Chem. 73:915). Toluene-filled multiwalled carbon nanotubes as a film on a gold electrode surface were shown to respond better to electroactive biomolecules than empty carbon nanotubes (Zhang et al. (2003) Electrochimica Acta 49:715).
In WO 2004/034025, a system to measure the redox potential is described that uses a potentiometric electrochemical system based on a metal-coated silicon nanowire.
There is a need for a nanoscale detection system that does not require the binding of a binding agent and target analyte directly to the detecting nanowire or CNT. Applicants have solved this problem by developing a single-walled carbon nanotube nanosensor that responds to a target analyte by altering the redox potential in solution, which in turn alters the redox state of the CNT and causes a change in its conductance. In addition to expanding the possibility of binding agent-target analyte pairs that may be used in a nanoscale detection system, this novel system removes the spatial limitation for close proximity of the binding agent and CNT.